LCDs are widely applied to monitors of personal computers, mobile devices, and TV sets in view of various advantages such as low voltage, low power consumption, and allowing for size and thickness reduction. Various modes have been proposed for LCDs according to the alignment of liquid crystal molecules in a liquid crystal cell. A twisted nematic (TN) mode has been the mainstream, in which the orientation direction of liquid crystal molecules is twisted from one side to the other side of the cell at about 90°.
An LCD is generally composed of a liquid crystal cell, an optical compensation film, and a polarizer. The optical compensation film, which serves to eliminate image discoloration or widen the viewing angle, includes a stretched birefringent film and a transparent film coated with a liquid crystal compound. Japanese Patent 2587398 discloses a viewing angle widening technique using, in a TN mode liquid crystal cell, an optical compensation film prepared by applying a discotic liquid crystal compound to a triacetyl cellulose film, aligning the liquid crystal molecules, and fixing the alignment. Even such a latest technique is still unsatisfactory, however, in applications to LCD TVs that are expected to have a wide screen and be seen from wide angles and therefore strictly required to have reduced viewing angle dependence. Under this situation, LCD modes different from the TN mode have been studied, including an IPS (in-plane switching) mode, an OCB (optically compensatory bend) mode, and a VA (vertically aligned) mode. In particular, a VA mode is attracting attention for application to TV monitors because of its high contrast and relatively high production yield.
In general, polyvinyl alcohol (hereinafter, PVA) is mostly used as a material of a polarizer essential to LCDs. PVA film is uniaxially stretched followed by dyeing with iodine or a dichroic dye, or dyeing may precede stretching, further followed by crosslinking with a boron compound to make a polarizer (polarizing film).
A cellulose acylate film is characterized by having higher optical isotropy (a low retardation value) compared with other polymer films. It is therefore common to use a cellulose acylate film in applications demanding optical isotropy such as a protective film of a polarizing plate.
To the contrary, optical anisotropy (a higher retardation value) is required of the optical compensation film (retardation film). An optical compensation film for a VA mode, in particular, is required to have a front retardation (Re) of 30 to 200 nm and a thickness direction retardation (Rth) of 70 to 400 nm. Hence, a synthetic polymer film with a high retardation value, such as polycarbonate film or polysulfone film, has usually been used as an optical compensation film.
In short, it has been a general principle in the field of optical materials to use a synthetic polymer film where optical anisotropy (a high retardation value) is demanded and to use cellulose acylate film where optical isotropy (a low retardation value) is demanded.
Contrary to the general principle, EP 911656 proposes a cellulose acetate film having a high retardation value and useful in applications demanding optical anisotropy. According to the proposal, a high retardation value of a cellulose triacetate film is achieved by adding an aromatic compound having at least two aromatic rings, especially a compound having a 1,3,5-triazine ring, as a retardation increasing agent to cellulose triacetate and stretching the resulting film. Because cellulose triacetate is generally difficult to stretch, it is known difficult to increase the birefringence of cellulose triacetate by stretching. According to the EP, co-stretching the additive allows for increasing the birefringence to achieve a high retardation value. The resulting film also functions as a protective film of a polarizing plate, thereby providing a competitive, slim LCD.
JP-A-2002-71957 discloses an optical film containing a cellulose ester having acyl groups having 2 to 4 carbon atoms as substituents. The acyl group substituents satisfy relations: 2.0≦A+B≦3.0 and A<2.4, where A is the substitution degree of acetyl, and B is the substitution degree of propionyl or butyryl. The refractive index Nx in the slow axis direction and the refractive index Ny in the fast axis direction of the optical film, both at 590 nm, satisfy the relationship: 0.0005≦Nx−Ny≦0.0050.
JP-A-2003-270442 discloses a polarizing plate for use in a VA mode LCD. The polarizing plate comprises a polarizer and an optically biaxial, mixed fatty acid cellulose ester film. The polarizing plate is placed with the cellulose ester film between a liquid crystal cell and the polarizer.
JP-A-2002-333523 discloses a polarizing plate having an in-plane phase difference (Δnd) of 500 to 1000 nm and an LCD having the polarizing plate. The proposed polarizing plate exhibits high contrast characteristics and high dimensional stability due to reduced residual stress.
The above-described related art techniques are effective in providing inexpensive and thin LCDs. On the other hand, LCD technology has recently been rapidly increasing application to televisions. Since LCD-TVs have a brighter backlight than other LCD monitors, display unevenness is more outstanding. In particular, a wide-screen LCD-TV requires a wide and yet uniform screen properties. Therefore, a polarizing plate for these applications is required to have a wide area with uniform quality.
The currently available, wide polarizing plates lack uniformity of stretching, that is, they suffer uneven stretching. The uneven stretching is primarily attributed to uneven alignment of PVA. The unevenness can be evaluated in terms of variation of PVA's in-plane retardation Rpva that correlates to the PVA's degree of alignment.
The stretch alignment unevenness in PVA is apt to occur in the direction perpendicular to the stretching direction and appears as streaky unevenness in density when an LCD is lit from the back. Use of such a polarizing plate in a wide-screen LCD results in streaky defects on the screen. Hence, development of a polarizing plate with little stretch alignment unevenness in PVA has been demanded.
JP-A-2002-333523 supra specifies the in-plane retardation of a polarizing plate and proposes a process of producing a polarizing plate with sufficient optical characteristics but gives no considerations to uniformity of the in-plane retardation value.